Block copolymers of styrene and conjugated diolefins have long been known as useful as elastomeric thermoplastics. These block copolymers are referred to as thermoplastic polymers because they may be worked by heating the polymer to above the polymer's melting temperature, and then processing by such methods as vacuum forming, extrusion, compression molding, calendering or blow molding. These block copolymers have good tear resistance, flexibility, thermomechanical stability and other properties.
The thermoplastic properties of block copolymers of styrene and conjugated diolefins are the result of incompatibility between the polystyrene and the polydiolefin polymeric blocks which causes separate polymeric phases to exist. At service temperatures, the polydiolefin domains are rubbery and elastic, whereas the polystyrene domains are hard and glassy. The polystyrene domains serve as physical crosslinks between the rubbery polydiolefin blocks. This causes the polymer to behave much like a vulcanized rubber at temperatures which are below the polystyrene glass transition temperature. By heating the block copolymer to a temperature higher than the glass transition temperature of the polystyrene domains, the polymer may be processed as a melt and formed into useful shapes.
Although these polymers are processable in commercial polymer handling equipment and have many excellent properties, they have short-comings. The glass transition temperature of the polystyrene domains limit the polymer's maximum service temperature. Further, these polymers are not compatible with polar engineering thermoplastics, causing difficulty in making polymeric blends with these materials. Adhesion to polar substrates and polar coatings is also generally not good, and solvent resistance of these polymers is deficient.
It has been found that these shortcomings can be partially overcome by incorporating polar functional groups into the polymers. It has been found to be particularly advantageous to incorporate metal salt functional groups into the polystyrene blocks. Ionic bonds between metal salt groups increase the glass transition temperatures of the polystyrene phases and render the polystyrene phases significantly less soluble in nonpolar solvents. The ionic bonds also increase the mechanical integrity of the polystyrene domains. As a result, the polymer's solvent resistance, high temperature properties and tensile strength are significantly increased by incorporating metal salt functionality into the polystyrene domains. Polymers which include metal salt functionality are commonly referred to as ionomers. Incorporation of metal salt of carboxylic acid functionality into the styrene blocks of such a polymer may increase the service temperature to 120.degree. C. or higher. The same block copolymer without functionality is limited to service temperatures of about 100.degree. C.
Unfortunately, incorporation of metal salt functionality into styrene blocks of these block copolymers is detrimental to processability. The ionic interactions which cause the improvements in the block copolymer's mechanical properties at service temperatures remain active above the glass transition temperature of the polystyrene domains. These ionic interactions interfere with the processability of the polymer melts by raising the viscosities of the melts. Typical plasticizers, such as processing oils, glycerols, dioctyl phthalate, diethyl phthalate, and dioctyl succinate, improve processability of styrene block functionalized styrene-butadiene block copolymers, but only at the expense of many of the physical properties which the functionalization is intended to improve. These plasticizers are liquids at polymer service temperatures, and as such they tend to decrease the glass transition of the styrene domains, decrease the composition's modulus and decrease the composition's tensile strength.
Other ionomers are also difficult to process, and plasticizers for some of them have been developed. Terpolymers of ethylene-propylene and diolefins, such as EPDM, which are sulfonated and then neutralized with metal ions to produce salt functionality are commercially available. The ionic crosslinks of these polymers serve many of the same functions as the styrene blocks of the styrene-conjugated diene block copolymers. The ionic crosslinks tie polymer chains together which results in properties like vulcanized rubber, but the ionic crosslinks are broken at elevated temperatures with the aid of a plasticizer. Preferred plasticizers for salts of sulfonated EPDM include zinc stearate and aliphatic organic amides. The use of these plasticizers with ionomers of EPDM is taught in U.S. Pat. Nos. 3,847,854 and 4,137,203. These plasticizers are referred to as ionic plasticizers because they function by relaxing ionic bonds, as opposed to backbone plasticization which is accomplished by enhancing slippage between polymeric backbones.
Zinc stearate and other metal salts of fatty acids are structurally similar to polymers such as EPDM and hydrogenated polyconjugated diolefins and would therefore by expected to function effectively as plasticizers for ionomers of these polymers. U.S. Pat. No. 4,137,203 demonstrates that zinc stearate is not only an excellent plasticizer for EPDM, but at concentrations between about 10 and 60 parts by weight per 100 parts by weight of polymer it doubles as an excellent reinforcing filler. These compositions have tensile strengths at room temperature which are up to four times the raw polymer tensile strength.
As opposed to EPDM and hydrogenated polyconjugated diolefins, fatty acids and metal salts thereof are incompatible with polymerized vinyl aromatics such as polystyrene. When acids or metal salts thereof are combined with ionomers of hydrogenated block copolymers of vinyl aromatics and conjugated diolefins, it is expected that the fatty acids would partition to the conjugated diolefin domains. If the fatty acids or metal salts thereof are predominately in the conjugated diolefin domains, it is not expected that they would be effective in plasticization of the ionomeric links in the poly(vinyl aromatic) domains.
The present inventors have surprisingly found that salts of fatty acids are effective plasticizers for vinyl aromatic block functionalized block copolymers of vinyl aromatics and conjugated diolefins. Further, they have also surprisingly found that metal salts of fatty acids are effective as reinforcing fillers when incorporated into these polymeric compositions.
It is therefore an objective of the present invention to provide a plasticizer for functionalized block copolymers, the functionalized block copolymer comprising at least one vinyl aromatic block and at least one conjugated diolefin olefin block in which the functionality comprises acid or metal salt functionality incorporated in the vinyl aromatic blocks wherein the plasticizer does not significantly reduce the 70.degree. C. properties of the polymer and which acts as a reinforcing filler at temperatures at or below 70.degree. C. In another aspect, it is an objective of this invention to provide a composition comprising such a functionalized block copolymer and a plasticizer which may be processed at temperatures typical of commercial polymer melt processing equipment.